1. Field of the Invention
The present invention relates generally to a three junction SQUID (superconducting quantum interference device), and more particularly pertains to a three junction SQUID mixer circuit wherein a nonlinear element such as a Josephson junction is coupled with a dc SQUID formed by two other nonlinear elements such as Josephson junctions and two input coils to provide a mixer circuit that can be employed for mixing two input magnetic flux signals. The mixer can be used for up-converting very low frequency signals, or for down-converting high frequency signals, in order to minimize and solve problems associated with excess noise in high resolution or quantum noise limited SQUID systems.
2. Discussion of the Prior Art
The present invention solves a significant problem associated with excess noise in the detection of magnetic flux signals, as might be present in applications in biomagnetism and in the military. No efficient alternative solution is known for minimizing and solving problems associated with excess noise, as discussed hereinbelow, in the detection of magnetic flux signals in high resolution or quantum noise limited SQUID systems.
U.S. Pat. No. 4,117,354 entitled "Two-Part Current Injection Interferometer Amplifiers and Logic Circuits" discloses Josephson junction interferometers having nonlinear switching or threshold characteristics. The nonlinear threshold characteristic is achieved in a preferred manner by applying an injection current to the interferometer at a point in the interferometer which is different from where its gate current is normally applied. The resulting nonlinearity provides for high amplification. The nonlinear switching characteristic may also be achieved by applying an injection current to the same point on the interferometer where the gate current is normally applied. However, a portion of the applied injection current is electromagnetically coupled to the interferometer inductance to achieve a desired nonlinear switching characteristic.
U.S. Pat. No. 4,117,503 entitled "Josephson Interferometer Structure which Suppresses Resonances" discloses that Josephson interferometers with inductive, capacitive and resistive components are subject to the presence of relatively high amplitude resonances similar to those found in in-line gases. Interferometer structures exhibit the same resonant behavior as long tunnel junctions, except that there exist only as many discrete resonance voltages as meshes in the interferometer device. Hence a two junction interferometer has one resonance as compared to two resonances in a three junction device. In the I-V characteristic of a Josephson tunneling device, such as an interferometer, such resonances appear as current steps which must be taken into account in the design of Josephson switching circuits primarily to avoid a situation wherein the load line of an external load intersects a resonance peak. Where the load line and the resonance peak intersect because such an intersection is stable, the device is prevented from switching to the full voltage desired. Such resonances can be effectively suppressed in interferometers by providing a resistance which is in parallel with the main inductance of the interferometer. In a two junction interferometer, the resistance is effectively connected between the base electrode metallizations which are utilized to form one of the electrodes of each of the pair of electrodes required for each interferometer junction. To the extent that more than two junctions are utilized, the resonance suppressing resistor is connected between pairs of junctions and across the main inductances which interconnect the junctions. The structure of a two junction interferometer with a resonance-suppressing resistor is shown as well as the schematics of a multiple junction interferometer which clearly indicates how such structures may be fabricated.
U.S. Pat. No. 4,916,335 entitled "Superconducting Circuit" describes a quantum flux parametron-type superconducting circuit in which a path with a Josephson device is connected in parallel with exciting inductors of the quantum flux parametron or inductors of exciting line magnetically coupled with the exciting inductors, thereby constructing a phase regulator.
Japanese Patent 63-261885 entitled "Three-Junction DC SQUID Gate" describes a superconducting closed circuit constituted of first, second and third branch lines which are connected to a gate current line at both ends and in parallel with each other. The first branch line includes a series circuit of an inductance and a Josephson having a critical current value. The second branch line includes only a Josephson element having a second critical current value. The third branch line includes a series circuit of an inductance and a Josephson element having a third critical current value. The inductances constitute an asymmetrical three junction DC SQUID gate coupled with a control current line by magnetic induction. A sufficiently wide operation margin can be obtained while high gain or high sensitivity is maintained.
European Patent Application 0 076 160 entitled "Josephson Junction Logic Device" discloses a Josephson junction logic device comprising at least first and second superconductive loops and at least two logic input signal lines, with each of the logic input signal lines being arranged to be opposite to one of the superconductive loops so that each logic input signal line is able to magnetically coupled with only said one of the superconductive loops and is magnetically independent from the other superconductive loop, whereby the allowable range for maintaining the device in a superconductive state is expanded.
IBM Technical Disclosure Bulletin 21062534 by P. Gueret entitled "New Type of Single Flux Quantum Cell with Large Margins" describes a center-feed three junction interferometer having junctions in series with a center inductance.
IBM Technical Disclosure Bulletin 274A2157 by S. B. Kaplan describes a multi-junction logic device using two superconductive loops with three Josephson junctions and two input signal lines. Kaplan also discloses a similar SQUID having only a single input coil in which the circuit uses an input signal in the input coil to switch the SQUID from a zero voltage state into a voltage state. This is a common feature in all of the prior art references which are aimed at computer switching and memory circuit applications in which one of the two inputs is in the form of a flux linked into the SQUID through a coil, and the other input is applied directly to the SQUID junctions through a gate current. The three Josephson junction arrangement disclosed herein is similar to that of the present invention, but Kaplan does not disclose or teach or even consider the use of the arrangement as a mixing device for up-converting very low frequency signals, thereby minimizing and solving problems with low frequency noise limitations, or for down-converting high frequency signals.
IBM Technical Disclosure Bulletin 25062940 by W. H. Chang describes a three junction Josephson interferometer with two superconducting loops which have a slight mismatch in their LIo products to provide a slightly asymmetric interferometer.